A wireless local area network (WLAN) has been more and more widely used in the last few years. Originally, WLAN was designed to eliminate tangled cables among network devices in an indoor environment, and handoff support was not perceived as a critical issue. However, with the fast development of the WLAN technology, in practical applications, the WLAN is expected not only to provide a high data rate, but also to support real-time services, such as Voice over Internet Protocol (VoIP) and multimedia applications. As the real-time services have high requirements for time delay, the WLAN technology needs to support fast handoff between different access points (APs). However, due to insufficient handoff support in a WLAN network based on the IEEE802.11 Protocol, a significant disruption can be experienced while a mobile host (MH) performs handoff. Consequently, support of fast smooth handoff in a WLAN network has become a vital issue to achieve seamless mobile services.
In a WLAN network, a whole handoff procedure can usually be divided into three phases: scanning, authentication, and reassociation. In the scanning phase, the IEEE802.11 standard provides two types of scanning schemes: passive and active. In the passive scanning, MHs monitor each channel one by one for beacon frames sent by APs. In active scanning, MHs transmit probe requests on each channel, and the APs that receive the probe request feed back probe response frames. After all channels are scanned, the MH selects one AP for association according to the feedback information from all available APs. It is obvious that in both active and passive scanning schemes, the scanning phase in the handoff procedure leads to great delay.
In order to solve above problem and realize fast handoff in the WLAN network, many solutions are proposed.
For example, in order to reduce delays that occurs in the scanning procedure, a non-overlapping neighbor graph technology is proposed. In this technology, a non-overlapping neighbor graph for each channel of each AP needs to be constructed. If the MH cannot communicate with two APs with an acceptable communication quality at the same time, the two APs are non-overlapped. When the MH receives a probe response of an AP, it can be judged that no probe response from other APs in the non-overlapping neighbor graph of the AP is to be received, so the probe responses of the APs need not to be waited for. Therefore the number of probe responses to be waited for is reduced, so that time required for scanning is saved. A critical point of the technology is to acquire a non-overlapping neighbor graph, but currently acquisition of the non-overlapping neighbor graph is very difficult, so implementation of the technology is quite complicated. Further, as the technology needs to modify existing handoff protocols, the implementation cost is too high for current networks.
In order to reduce delays during the scanning procedure, a SyncScan technology is further proposed. In the technology, the MH is required to be handed off to each channel regularly and monitor nearby APs. Therefore, when handoff is required, the AP to which the MH can be handed off can be acquired without scanning. The SyncScan technology requires the MH to perform periodic scanning, so the processing of the MH is increased, and other work of the MH is influenced.
Above handoff technologies for WLAN network need to change handoff protocols, so the implementation is quite complicated, the cost is too high, and practical implementation is very difficult to be performed to realize fast handoff of the WLAN network.